In the oil and gas industry, the fluids extracted from a well often consist of a mixture of gas and liquid. Separation of gas and liquid phases is normally carried out using conventional gravity separators. These are large horizontal settlement tanks into which the fluids are fed and allowed to settle under gravity, the liquids gradually sinking to the bottom of the tank while the gases rise to the top. The gases and liquids can then be drawn off separately. The separation of gas and liquid phases may be carried out in more than one stage, with each stage having a different operating pressure.
The main disadvantages of conventional gravity separators are their size and large fluid inventory, which is an important safety issue in case of explosion or fire. A saving in space and weight is therefore an important issue, particularly on offshore platforms. For this reason compact separators offer significant savings in the cost of the process systems, as well as improved safety.
Compact separators that can offer the desired saving in size and weight are generally of the cyclonic type and generate high “g” forces to improve separation efficiency and reduce the need for a long residence time, which is needed for normal gravity separators. Certain compact separators are described for example in international patent applications WO95/07414 and WO2004/083601. Such separators work very efficiently in ideal conditions, with a steady fluid flow rate and a consistent mixture of gases and liquids.
However, in many cases the flow of multi-phase fluids entering the separator is erratic and variable in terms of both consistency and the instantaneous flow rate of the gas and liquid phases. These characteristics of the flowing fluids are known as the “flow regime” and are dictated by a number of factors including the flow rate of gas and liquid phases, the operating pressure and the size, profile and length of the pipeline that carries the flow into the separator. In some cases the flow regime is of slug form or semi-slug form, meaning that the instantaneous flow entering the separator has a substantial volume of the liquid phase followed by gas. In each case the instantaneous flow rate could at times be several times that quoted as the average daily rate.
One further shortcoming of most cyclonic separators is their limit for turn-down operation. This is the minimum flow rate required for efficient operation. Typically, if the mixture flow rate drops below one fifth of the designed flow rate, the performance efficiency of the unit drops because it can no longer generate sufficiently high “g” forces for efficient cyclonic separation of the liquid and gas phases.
Fluctuations in the flow rate affect the efficiency of all separators, but compact cyclonic separators are even more sensitive to variable flow regimes because of their compactness. The flow regimes normally experienced cause the instantaneous flow rates of the gas and liquid flowing into the compact separator to vary significantly. The result is the carry-over of some gas in the separated liquid phase and the carry-over of some liquid in the separated gas phase. The amount of carry-over in each phase is dictated by the severity of the variations in the flow regime and the design of the compact cyclonic separator.
A cyclonic separator is described in international patent application No. WO99/22873A. The device is designed primarily for separating dust particles from air in a vacuum cleaner, although it may also be used for separating mixtures of gases and liquids. If multi-phase fluids are fed into the separator a vortex is created, causing centrifugal separation of the denser fluids from the less dense fluids. The denser fluids (primarily liquids) move towards the outer wall of the separator and leave through a tangential outlet vent whereas the less dense fluids (primarily gases) move inwards and leave through an axial outlet vent. However, complete separation of gases and liquids is rarely achieved. Usually, the denser fluids include some gas in addition to the liquid and the less dense fluids include some droplets of liquid along with the gas.
Another separation apparatus is described in WO 2004/083601 A. This includes a cyclonic separator having a first outlet primarily for gases and a second outlet primarily for liquids. The first outlet is connected to the inlet of a conventional knock-out vessel, which is designed to remove droplets of liquid from the separated gas. The droplets fall into a body of liquid in the bottom of the vessel and are recombined with the fluids (primarily liquids) flowing through the second outlet of the cyclonic separator, while the cleaned gas is drawn off from the top of the knock-out vessel. This apparatus therefore reduces the amount of liquid carried over in the gas phase, but does not prevent gases from being carried over in the liquid phase. In certain applications this may be unacceptable. Furthermore, because the separation process is carried out mainly by the cyclonic separator, the apparatus does not address the problems caused by variable flow regimes, which may prevent efficient operation of the cyclonic separator.
Another gas/liquid separator described in GB 2191424 A includes a centrifugal separator mounted within a horizontal gravity separator. The centrifugal separator is a dual hydrocyclone in which the separated fluids flow in opposite directions to axially separated outlets. The gas fraction is discharged into the upper part of the gravity separator, where demisters and turbulence generators are provided to coalesce and remove aerosolised liquid droplets from the gas flow.
It is an object of the present invention to provide a method and an apparatus for separating multi-phase fluids, which mitigates at least some of the aforesaid disadvantages.